In a digital transmission system, such as a direct sequence spread spectrum communication system, a biphase shift-keyed modulator (hereinafter BPSK modulator) is used to modulate a radio frequency (RF) transmission carrier with binary information. The BPSK modulator selects for transmission either an in-phase (i.e., 0-degree) or an out-of-phase (i.e., 180-degree phase shifted) representation of the carrier signal, in response to a control signal which usually corresponds to the digital bits (i.e., binary information) to be sent.
The usual apparatus for accomplishing this task involves the use of two center-tapped transmission line transformers and a matched set of four switching diodes. This arrangement is commonly known as a ring modulator. In operation, the ring modulator routes the chosen phase of the carrier signal to the output by forward biasing a selected pair of the switching diodes, causing a DC switching current, and the desired phase of the AC carrier signal, to flow through the selected pair of diodes. A suitable diode array for use in a ring modulator is the SBL-1 ring diode array manufactured by Mini-Circuits, Inc., Brooklyn, N.Y.
It is noted that, in general, ring modulators perform their signal switching functions quite well, but unfortunately, ring modulators have a disadvantage that a relatively large DC switching current (which may be in excess of 20 milliamperes) is required to ensure that the switching diodes are fully turned on.
In a battery-operated digital transceiver, such as a 900 MHz direct sequence spread spectrum cordless telephone handset, conservation of battery power is of utmost concern. In this environment, the 20 milliampere DC switching current of conventional ring modulators, mentioned above, is considered an unacceptable drain on the limited battery resources available.